X-ray tubes that are used in this type of radiographic equipment have structures that produce x-rays through collisions with an anode of thermal electrons that are from a cathode that is provided with a filament. Given this, when the electric current value of the filament current that is provided to the filament becomes large, more thermal electrons will be emitted from the cathode toward the anode, increasing the value of the tube current, to thereby cause the emission of a larger dose of x-rays.
In such an x-ray tube, the filament of the x-ray tube degrades down over time. Moreover, it is known that when the filament of the x-ray tube undergoes degradation, the tube current in the x-ray tube will become larger, even when the filament current that is provided to the filament is held constant. Thus when a filament that breaks down with the passage of time is used with the same filament current, the value of the tube current will become larger, not only producing x-rays of a dose that is larger than what is necessary, but also increasing the temperature of the filament and increasing the amount of heat produced by the filament, which further causes the filament to become even narrower, accelerating the rate with which it undergoes degradation.
Because of this, measuring the actual value of the tube current of the x-ray tube when emitting x-rays, and performing feedback control of the value of the electric current for the filament current accordingly, makes it possible to maintain the tube current at a constant value. However, in an x-ray tube the response speed of the filament temperature relative to the value of the filament current that is applied to the filament is relatively slow, so a some amount of time is required in order to stabilize the tube current value and the tube voltage value. Because of this, unless x-rays are emitted over a relatively long period of time, in order to perform such feedback control, it is not possible to perform feedback control accurately through differences in the tube current value, or the like.
Because of this, in the x-ray generating device set forth in Japanese Unexamined Patent Application Publication H9-161990 (“JP '990”), an initial value for the data that indicates the relationship between the tube current value and the filament current value, and data that indicates the relationship between the tube current value and the filament current value at the present point in time, are compared, to calculate the value of the filament current for producing the required tube current value.
While the tube current value will vary over time when the filament current value is constant, the tube current value undulates over time. Because of this, as described in JP '990, when the filament current value has been corrected using the measured value at a given point in time as-is, the variability of differences in the measured values will be reflected as-is into the correction values, so that the corrections will not always be accurate. Moreover, even when there is error in the measured value, that error will be reflected as-is into the correction value.
The present invention was created in order to resolve issues such as set forth above, and the object thereof is to provide radiographic equipment wherein variability in the tube current values accompanying degradation of the filament in an x-ray tube can be corrected appropriately by reducing the effects of variability of differences over time and of measurement error.
Moreover, in such radiographic equipment, the filament breaking at the time of an x-ray examination, preventing the x-rays from being emitted from the x-ray tube, may endanger the life or safety of the patient, depending on the condition of the patient that is the subject of the examination.
The present invention is to solve the problems set forth above, and a second object thereof is to provide radiographic equipment wherein it is possible to anticipate the failure of the radiographic equipment due to the service life of filament.
The invention as set forth is radiographic equipment for detecting, by an x-ray detector, x-rays that have been emitted from an x-ray tube and that have passed through an examination subject, including storing means for storing, as a filament current setting value, a filament current that is to be supplied to a filament of an x-ray tube under imaging conditions, from the imaging conditions of the tube current and tube voltage that are to be supplied to the x-ray tube at the time of radiographic imaging; tube current value difference measuring means for measuring over time, as a tube current value difference, a difference between an anticipated value for the tithe current when x-ray emission is performed at a given filament current and a measured value for the tube current when x-ray emission is actually performed at that filament current; and filament current setting value correcting means for correcting the filament current setting value that has been stored in the storing means so that the tube current value difference is reduced when a calculation processing value for the tube current value difference during a specific period exceeds a setting value that has been set in advance.
The invention as set forth is radiographic equipment as set forth above, wherein: the calculation processing value for the tube current difference during the specific interval is a mean value or a weighted average value of the tube current difference.
The invention also includes warning displaying means for displaying a warning regarding the filament service life when a correcting frequency for the filament current setting value exceeds a specific setting frequency in a given time interval.
An embodiment of the invention includes the filament current setting value correcting means correct the filament current setting value by a minimum correction value that is smaller than a correction value for the filament current corresponding to the current value difference when the calculation processing value for the tube current value difference during the specific interval exceeds a setting value that has been set in advance.
The radiographic equipment as set forth includes when the tube current value difference is a difference wherein the tube current value is increased, the filament current setting value correcting means making a correction to reduce the filament current setting value, and when the tube current value difference is a difference wherein the tube current value is increased, do not make a correction to the filament current setting value.
The inventions as make it possible to correct accurately variability in the tube current value accompanying degradation of the filament in an x-ray tube by reducing the effect of variability of differences over time and of measurement errors, through correcting the filament current setting value based on a calculation process value for the electric current value differences over a specific time interval.
The invention as set forth also makes it possible to anticipate in advance failures of the radiographic equipment that occurred due to the service life of the filament, through monitoring the state of corrections of the filament current setting value.
The examples above make it possible to correct the filament current setting value gradually, even when there is a variability in differences over time or when there is measurement error, through correcting the filament current setting value by a minimum correction value that is smaller than the correction value for the filament current corresponding to the electric current value difference.
The invention makes it possible to perform the correction in accordance with the degradation characteristics of the filament, and possible to prevent an excessive increase in the filament current value, through correcting the filament current value setting to be smaller when there is a tube current value difference wherein the tube current value has increased, and not correcting the filament current value setting when there is a tube current value difference wherein the tube current value has decreased.